1. Field of the Invention
The present invention relates to the manufacture of high sensitivity, high stability thin film gas sensors for the detection and measurement of gaseous hydrocarbon contaminants having double and triple bonds in air. The sensor employs semiconductive tungsten oxide on a substrate of a lithium niobate monocrystal. The electrical resistance of the tungsten oxide layer is measured as a function of the type and concentration of the gas to be detected.
2. Description of the Prior Art
A true selectivity for a specific gas is difficult to achieve due to the functional principle of gas sensors utilizing semiconductor metal oxides. Accordingly, some other measuring methods must be used, for example, gas chromatography or infrared absorption. In each instance, these measuring methods require a far higher expenditure of money and time. When, however, a gas sensor is to be employed only in alarm and measuring devices having lower precision for monitoring the occurrence of undesirable or dangerous gases, then the metal oxide gas sensors have distinct advantages because they can be applied to many measuring locations without great expense and can be simultaneously monitored.
Depending on the preparation, material selection, and the various additives present as well as on operating parameters, various sensitivities are obtained using metal oxide gas sensors for various gases.
A gas sensor of the type described is illustrated, for example, in European Patent Application No. 0 046 989 and can be employed for the detection and measurement of gaseous hydrocarbon contaminants in the air. Hydrocarbon gases such as methane, propane, butane, ethylene, acetylene and even hydrogen are generally detected with this gas sensor on the basis of the semiconductive tungsten oxide film on substrates of silica glass, oxidized silicon, or a ceramic. The particular sensitivity for specific hydrocarbon compounds has not been identified.
The European Patent Application No.0 046 989, the disclosure of which is hereby incorporated by reference, illustrates a structure which is applicable for the present invention. It consists of a substrate of silica glass, oxidized silicon or a ceramic. On the opposite side of the substrate, i.e., immediately below the substrate is a surface that is highly conducting and is provided with spaced metal contacts for direct heating.
The thin film of sputtered tungsten oxide appears immediately above the silicon dioxide layer. A layer of precious metal such as platinum appears over the tungsten oxide film as an activator. Electrodes are bonded to the thin film of tungsten oxide, spaced by the layer of precious metal.